Reactivities of Silicas with Organometallic Methylating Agents

Abstract

The surface of silica gel was modified by a variety of procedures aimed at the introduction of CH3−Si moieties. The formation of these moieties by reactions of either SOCl2-chlorinated silica or SiCl4-modified silica with simple organometallic reagents (methyllithium, methylmagnesium bromide, dimethylzinc, or trimethylaluminum) was examined. Solid-state 29Si and 13C NMR techniques were used to characterize the resulting materials, especially to determine the extent of methylation of the silica surface. It was shown that, for a synthetic pathway that includes a chlorination step, methylmagnesium bromide was the best reagent for simply attaching a methyl group to a surface silicon atom. Methyllithium is very destructive to the silica framework, and trimethylaluminum forms a very small amount of Si−CH3 moieties when allowed to react with silica itself, SOCl2-chlorinated silica, or SiCl4-treated silica. There was no obvious evidence that dimethylzinc reacted with dry silica or with either SOCl2-modified or SiCl4-modified silicas to form Si−CH3 bonds.